Aerodynamic drag along the external surfaces of an aircraft decreases aircraft efficiency and requires the use of increased fuel in flight. It is a long standing premise of aircraft design that reduced drag increases aircraft efficiency and range. A significant part of aircraft drag originates with the generation of a boundary layer across aircraft external surfaces. Generally, the boundary layer is a layer of fluid, in this case air, that moves at a retarded speed immediately adjacent to the aircraft skin (surface). This is because frictional forces retard the motion of the fluid in a thin layer near the surface. As the aircraft passes through the air stream, this thin boundary layer, in which friction must be taken into account, increases in thickness considerably in the downstream direction, such that flow in the boundary layer may become reversed. At this point the boundary layer separates from the surface. This is what is called boundary layer separation. This phenomenon is always associated with the formation of vortices and very large energy losses in the wake of the aircraft body. The large drag of such aircraft bodies can be explained by the existence of the large deviation in pressure distribution which is a consequence of boundary layer separation.
It has therefore been a tenet of aircraft design to minimize boundary layer generation across aircraft wings and fuselages. While these boundary layers are impossible to completely eliminate, various means have been used to minimize their effect on aircraft performance. One method has been to physically remove the boundary layer through the use of suction ports on the wings or airframe that divert this thin layer sufficiently to prevent boundary layer separation. The use of suction to remove boundary layer can be done by at least two methods. The suction can be developed by connecting suction ports to outflow ports on the suction side (low pressure) of aircraft wings or suction pumps can be used to develop a low pressure relative vacuum source for the removal of the boundary layer. Unfortunately, it is not always possible to develop sufficient suction from the suction side of the wing without adversely affecting aircraft design. Further, use of suction pumps for boundary layer control requires an energy expenditure that adversely affects aircraft efficiency. This is particularly true with long-endurance aircraft, which are intended to operate efficiently for relatively long periods of time. Any boundary layer control system for long endurance aircraft with significant energy expenditures would have an adverse effect on fuel requirements.
A need exists, therefore, for a boundary layer control system which is effective in reducing aircraft drag and yet has minimal energy requirements.